Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapters.

Chapter XII.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to quite different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. I well remember, when first collecting in the fresh waters of
Brazil, feeling much surprise at the similarity of the fresh-water insects,
shells, &c., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having become
fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to
wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to fish, I
believe that the same species never occur in the fresh waters of distant
continents. But on the same continent the species often range widely and
almost capriciously; for two river-systems will have some fish in common
and some different. A few facts seem to favour the possibility of their
occasional transport by accidental means; like that of the live fish not
rarely dropped by whirlwinds in India, and the vitality of their ova when
removed from the water. But I am inclined to attribute the dispersal of
fresh-water fish mainly to slight changes within the recent period in the
level of the land, having caused rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent geological
period, and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted river-systems
and completely prevented their inosculation, seems to lead to this same
conclusion. With respect to allied fresh-water fish occurring at very
distant points of the world, no doubt there are many cases which cannot at
present be explained: but some fresh-water fish belong to very ancient
forms, and in such cases there will have been ample time for great
geographical changes, and consequently time and means for much migration.
In the second place, salt-water fish can with care be slowly accustomed to
live in fresh water; and, according to Valenciennes, there is hardly a
single group of fishes confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group might travel far along
the shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds, and they are immediately killed by sea water, as are
the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed -- and no doubt many others remain to be observed -- throw some light
on this subject. When a duck suddenly emerges from a pond covered with
duck-weed, I have twice seen these little plants adhering to its back; and
it has happened to me, in removing a little duck-weed from one aquarium to
another, that I have quite unintentionally stocked the one with fresh-water
shells from the other. But another agency is perhaps more effectual: I
suspended a duck's feet, which might represent those of a bird sleeping in
a natural pond, in an aquarium, where many ova of fresh-water shells were
hatching; and I found that numbers of the extremely minute and just hatched
shells crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched molluscs,
though aquatic in their nature, survived on the duck's feet, in damp air,
from twelve to twenty hours; and in this length of time a duck or heron
might fly at least six or seven hundred miles, and would be sure to alight
on a pool or rivulet, if blown across sea to an oceanic island or to any
other distant point. Sir Charles Lyell also informs me that a Dyticus has
been caught with an Ancylus (a fresh-water shell like a limpet) firmly
adhering to it; and a water-beetle of the same family, a Colymbetes, once
flew on board the Beagle, when forty-five miles distant from the nearest
land: how much farther it might have flown with a favouring gale no one
can tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have only a
very few aquatic members; for these latter seem immediately to acquire, as
if in consequence, a very wide range. I think favourable means of
dispersal explain this fact. I have before mentioned that earth
occasionally, though rarely, adheres in some quantity to the feet and beaks
of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds of
this order I can show are the greatest wanderers, and are occasionally
found on the most remote and barren islands in the open ocean; they would
not be likely to alight on the surface of the sea, so that the dirt would
not be washed off their feet; when making land, they would be sure to fly
to their natural fresh-water haunts. I do not believe that botanists are
aware how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I took
in February three table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond; this mud when dry weighed only
6 3/4 ounces; I kept it covered up in my study for six months, pulling up
and counting each plant as it grew; the plants were of many kinds, and were
altogether 537 in number; and yet the viscid mud was all contained in a
breakfast cup! Considering these facts, I think it would be an
inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to vast distances, and if consequently the range of
these plants was not very great. The same agency may have come into play
with the eggs of some of the smaller fresh-water animals.

Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected in
pellets or in excrement, many hours afterwards. When I saw the great size
of the seeds of that fine water-lily, the Nelumbium, and remembered Alph.
de Candolle's remarks on this plant, I thought that its distribution must
remain quite inexplicable; but Audubon states that he found the seeds of
the great southern water-lily (probably, according to Dr. Hooker, the
Nelumbium luteum) in a heron's stomach; although I do not know the fact,
yet analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds might
be dropped by the bird whilst feeding its young, in the same way as fish
are known sometimes to be dropped.

In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance, on a rising
islet, it will be unoccupied; and a single seed or egg will have a good
chance of succeeding. Although there will always be a struggle for life
between the individuals of the species, however few, already occupying any
pond, yet as the number of kinds is small, compared with those on the land,
the competition will probably be less severe between aquatic than between
terrestrial species; consequently an intruder from the waters of a foreign
country, would have a better chance of seizing on a place, than in the case
of terrestrial colonists. We should, also, remember that some, perhaps
many, fresh-water productions are low in the scale of nature, and that we
have reason to believe that such low beings change or become modified less
quickly than the high; and this will give longer time than the average for
the migration of the same aquatic species. We should not forget the
probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide
distribution of fresh-water plants and of the lower animals, whether
retaining the same identical form or in some degree modified, I believe
mainly depends on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have large powers of flight,
and naturally travel from one to another and often distant piece of water.
Nature, like a careful gardener, thus takes her seeds from a bed of a
particular nature, and drops them in another equally well fitted for them.

On the Inhabitants of Oceanic Islands. -- We now come to the last of the
three classes of facts, which I have selected as presenting the greatest
amount of difficulty, on the view that all the individuals both of the same
and of allied species have descended from a single parent; and therefore
have all proceeded from a common birthplace, notwithstanding that in the
course of time they have come to inhabit distant points of the globe. I
have already stated that I cannot honestly admit Forbes's view on
continental extensions, which, if legitimately followed out, would lead to
the belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in regard to
insular productions. In the following remarks I shall not confine myself
to the mere question of dispersal; but shall consider some other facts,
which bear on the truth of the two theories of independent creation and of
descent with modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large size
and varied stations of New Zealand, extending over 780 miles of latitude,
and compare its flowering plants, only 750 in number, with those on an
equal area at the Cape of Good Hope or in Australia, we must, I think,
admit that something quite independently of any difference in physical
conditions has caused so great a difference in number. Even the uniform
county of Cambridge has 847 plants, and the little island of Anglesea 764,
but a few ferns and a few introduced plants are included in these numbers,
and the comparison in some other respects is not quite fair. We have
evidence that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it, as
they have on New Zealand and on every other oceanic island which can be
named. In St. Helena there is reason to believe that the naturalised
plants and animals have nearly or quite exterminated many native
productions. He who admits the doctrine of the creation of each separate
species, will have to admit, that a sufficient number of the best adapted
plants and animals have not been created on oceanic islands; for man has
unintentionally stocked them from various sources far more fully and
perfectly than has nature.

Although in oceanic islands the number of kinds of inhabitants is scanty,
the proportion of endemic species (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the number
of the endemic land-shells in Madeira, or of the endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the islands with that of the continent, we shall see
that this is true. This fact might have been expected on my theory, for,
as already explained, species occasionally arriving after long intervals in
a new and isolated district, and having to compete with new associates,
will be eminently liable to modification, and will often produce groups of
modified descendants. But it by no means follows, that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds, are
peculiar; and it is obvious that marine birds could arrive at these islands
more easily than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess one
endemic land bird; and we know from Mr. J. M. Jones's admirable account of
Bermuda, that very many North American birds, during their great annual
migrations, visit either periodically or occasionally this island. Madeira
does not possess one peculiar bird, and many European and African birds are
almost every year blown there, as I am informed by Mr. E. V. Harcourt. So
that these two islands of Bermuda and Madeira have been stocked by birds,
which for long ages have struggled together in their former homes, and have
become mutually adapted to each other; and when settled in their new homes,
each kind will have been kept by the others to their proper places and
habits, and will consequently have been little liable to modification.
Madeira, again, is inhabited by a wonderful number of peculiar land-shells,
whereas not one species of sea-shell is confined to its shores: now,
though we do not know how seashells are dispersed, yet we can see that
their eggs or larvae, perhaps attached to seaweed or floating timber, or to
the feet of wading-birds, might be transported far more easily than
land-shells, across three or four hundred miles of open sea. The different
orders of insects in Madeira apparently present analogous facts.

Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take the
place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
shown that the proportional numbers of the different orders are very
different from what they are elsewhere. Such cases are generally accounted
for by the physical conditions of the islands; but this explanation seems
to me not a little doubtful. Facility of immigration, I believe, has been
at least as important as the nature of the conditions.

Many remarkable little facts could be given with respect to the inhabitants
of remote islands. For instance, in certain islands not tenanted by
mammals, some of the endemic plants have beautifully hooked seeds; yet few
relations are more striking than the adaptation of hooked seeds for
transportal by the wool and fur of quadrupeds. This case presents no
difficulty on my view, for a hooked seed might be transported to an island
by some other means; and the plant then becoming slightly modified, but
still retaining its hooked seeds, would form an endemic species, having as
useless an appendage as any rudimentary organ, -- for instance, as the
shrivelled wings under the soldered elytra of many insular beetles. Again,
islands often possess trees or bushes belonging to orders which elsewhere
include only herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an herbaceous
plant, though it would have no chance of successfully competing in stature
with a fully developed tree, when established on an island and having to
compete with herbaceous plants alone, might readily gain an advantage by
growing taller and taller and overtopping the other plants. If so, natural
selection would often tend to add to the stature of herbaceous plants when
growing on an island, to whatever order they belonged, and thus convert
them first into bushes and ultimately into trees.

With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have never
been found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and I have found it
strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this
exception (if the information be correct) may be explained through glacial
agency. This general absence of frogs, toads, and newts on so many oceanic
islands cannot be accounted for by their physical conditions; indeed it
seems that islands are peculiarly well fitted for these animals; for frogs
have been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we can
see that there would be great difficulty in their transportal across the
sea, and therefore why they do not exist on any oceanic island. But why,
on the theory of creation, they should not have been created there, it
would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found a
single instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island situated
above 300 miles from a continent or great continental island; and many
islands situated at a much less distance are equally barren. The Falkland
Islands, which are inhabited by a wolf-like fox, come nearest to an
exception; but this group cannot be considered as oceanic, as it lies on a
bank connected with the mainland; moreover, icebergs formerly brought
boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot
be said that small islands will not support small mammals, for they occur
in many parts of the world on very small islands, if close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that mammals
appear and disappear at a quicker rate than other and lower animals.
Though terrestrial mammals do not occur on oceanic islands, aerial mammals
do occur on almost every island. New Zealand possesses two bats found
nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all
possess their peculiar bats. Why, it may be asked, has the supposed
creative force produced bats and no other mammals on remote islands? On my
view this question can easily be answered; for no terrestrial mammal can be
transported across a wide space of sea, but bats can fly across. Bats have
been seen wandering by day far over the Atlantic Ocean; and two North
American species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who has
specially studied this family, that many of the same species have enormous
ranges, and are found on continents and on far distant islands. Hence we
have only to suppose that such wandering species have been modified through
natural selection in their new homes in relation to their new position, and
we can understand the presence of endemic bats on islands, with the absence
of all terrestrial mammals.

Besides the absence of terrestrial mammals in relation to the remoteness of
islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near
Celebes by a space of deep ocean; and this space separates two widely
distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied
or identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some
cases owing to the probable naturalisation of certain mammals through man's
agency; but we shall soon have much light thrown on the natural history of
this archipelago by the admirable zeal and researches of Mr. Wallace. I
have not as yet had time to follow up this subject in all other quarters of
the world; but as far as I have gone, the relation generally holds good.
We see Britain separated by a shallow channel from Europe, and the mammals
are the same on both sides; we meet with analogous facts on many islands
separated by similar channels from Australia. The West Indian Islands
stand on a deeply submerged bank, nearly 1000 fathoms in depth, and here we
find American forms, but the species and even the genera are distinct. As
the amount of modification in all cases depends to a certain degree on the
lapse of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously
united within a recent period to the mainland than islands separated by
deeper channels, we can understand the frequent relation between the depth
of the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent, -- an inexplicable relation
on the view of independent acts of creation.

All the foregoing remarks on the inhabitants of oceanic islands, -- namely,
the scarcity of kinds -- the richness in endemic forms in particular classes
or sections of classes, -- the absence of whole groups, as of batrachians,
and of terrestrial mammals notwithstanding the presence of aerial
bats, -- the singular proportions of certain orders of plants, -- herbaceous
forms having been developed into trees, &c., -- seem to me to accord better
with the view of occasional means of transport having been largely
efficient in the long course of time, than with the view of all our oceanic
islands having been formerly connected by continuous land with the nearest
continent; for on this latter view the migration would probably have been
more complete; and if modification be admitted, all the forms of life would
have been more equally modified, in accordance with the paramount
importance of the relation of organism to organism.

I do not deny that there are many and grave difficulties in understanding
how several of the inhabitants of the more remote islands, whether still
retaining the same specific form or modified since their arrival, could
have reached their present homes. But the probability of many islands
having existed as halting-places, of which not a wreck now remains, must
not be overlooked. I will here give a single instance of one of the cases
of difficulty. Almost all oceanic islands, even the most isolated and
smallest, are inhabited by land-shells, generally by endemic species, but
sometimes by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and are
killed by it. Yet there must be, on my view, some unknown, but highly
efficient means for their transportal. Would the just-hatched young
occasionally crawl on and adhere to the feet of birds roosting on the
ground, and thus get transported? It occurred to me that land-shells, when
hybernating and having a membranous diaphragm over the mouth of the shell,
might be floated in chinks of drifted timber across moderately wide arms of
the sea. And I found that several species did in this state withstand
uninjured an immersion in sea-water during seven days: one of these shells
was the Helix pomatia, and after it had again hybernated I put it in
sea-water for twenty days, and it perfectly recovered. As this species has
a thick calcareous operculum, I removed it, and when it had formed a new
membranous one, I immersed it for fourteen days in sea-water, and it
recovered and crawled away: but more experiments are wanted on this head.

The most striking and important fact for us in regard to the inhabitants of
islands, is their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this fact.
I will give only one, that of the Galapagos Archipelago, situated under the
equator, between 500 and 600 miles from the shores of South America. Here
almost every product of the land and water bears the unmistakeable stamp of
the American continent. There are twenty-six land birds, and twenty-five
of these are ranked by Mr. Gould as distinct species, supposed to have been
created here; yet the close affinity of most of these birds to American
species in every character, in their habits, gestures, and tones of voice,
was manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
archipelago. The naturalist, looking at the inhabitants of these volcanic
islands in the Pacific, distant several hundred miles from the continent,
yet feels that he is standing on American land. Why should this be so? why
should the species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact
there is a considerable dissimilarity in all these respects. On the other
hand, there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between the
Galapagos and Cape de Verde Archipelagos: but what an entire and absolute
difference in their inhabitants! The inhabitants of the Cape de Verde
Islands are related to those of Africa, like those of the Galapagos to
America. I believe this grand fact can receive no sort of explanation on
the ordinary view of independent creation; whereas on the view here
maintained, it is obvious that the Galapagos Islands would be likely to
receive colonists, whether by occasional means of transport or by formerly
continuous land, from America; and the Cape de Verde Islands from Africa;
and that such colonists would be liable to modification; -- the principle of
inheritance still betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the nearest
continent, or of other near islands. The exceptions are few, and most of
them can be explained. Thus the plants of Kerguelen Land, though standing
nearer to Africa than to America, are related, and that very closely, as we
know from Dr. Hooker's account, to those of America: but on the view that
this island has been mainly stocked by seeds brought with earth and stones
on icebergs, drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic plants is much more closely related to
Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enormously
remote, that the fact becomes an anomaly. But this difficulty almost
disappears on the view that both New Zealand, South America, and other
southern lands were long ago partially stocked from a nearly intermediate
though distant point, namely from the antarctic islands, when they were
clothed with vegetation, before the commencement of the Glacial period.
The affinity, which, though feeble, I am assured by Dr. Hooker is real,
between the flora of the south-western corner of Australia and of the Cape
of Good Hope, is a far more remarkable case, and is at present
inexplicable: but this affinity is confined to the plants, and will, I do
not doubt, be some day explained.

The law which causes the inhabitants of an archipelago, though specifically
distinct, to be closely allied to those of the nearest continent, we
sometimes see displayed on a small scale, yet in a most interesting manner,
within the limits of the same archipelago. Thus the several islands of the
Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
marvellous manner, by very closely related species; so that the inhabitants
of each separate island, though mostly distinct, are related in an
incomparably closer degree to each other than to the inhabitants of any
other part of the world. And this is just what might have been expected on
my view, for the islands are situated so near each other that they would
almost certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of the
islands may be used as an argument against my views; for it may be asked,
how has it happened in the several islands situated within sight of each
other, having the same geological nature, the same height, climate, &c.,
that many of the immigrants should have been differently modified, though
only in a small degree. This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering the
physical conditions of a country as the most important for its inhabitants;
whereas it cannot, I think, be disputed that the nature of the other
inhabitants, with which each has to compete, is at least as important, and
generally a far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other parts of
the world (laying on one side for the moment the endemic species, which
cannot be here fairly included, as we are considering how they have come to
be modified since their arrival), we find a considerable amount of
difference in the several islands. This difference might indeed have been
expected on the view of the islands having been stocked by occasional means
of transport -- a seed, for instance, of one plant having been brought to one
island, and that of another plant to another island. Hence when in former
times an immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly be
exposed to different conditions of life in the different islands, for it
would have to compete with different sets of organisms: a plant, for
instance, would find the best-fitted ground more perfectly occupied by
distinct plants in one island than in another, and it would be exposed to
the attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species
spreading widely and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous instances, is that the new species
formed in the separate islands have not quickly spread to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and sweep across
the archipelago, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they appear
to be on a map. Nevertheless a good many species, both those found in
other parts of the world and those confined to the archipelago, are common
to the several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take, I
think, an erroneous view of the probability of closely allied species
invading each other's territory, when put into free intercommunication.
Undoubtedly if one species has any advantage whatever over another, it will
in a very brief time wholly or in part supplant it; but if both are equally
well fitted for their own places in nature, both probably will hold their
own places and keep separate for almost any length of time. Being familiar
with the fact that many species, naturalised through man's agency, have
spread with astonishing rapidity over new countries, we are apt to infer
that most species would thus spread; but we should remember that the forms
which become naturalised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct species, belonging in
a large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, are distinct on each; thus
there are three closely-allied species of mocking-thrush, each confined to
its own island. Now let us suppose the mocking-thrush of Chatham Island to
be blown to Charles Island, which has its own mocking-thrush: why should
it succeed in establishing itself there? We may safely infer that Charles
Island is well stocked with its own species, for annually more eggs are
laid there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted for
its home as is the species peculiar to Chatham Island. Sir C. Lyell and
Mr. Wollaston have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of Porto Santo
possess many distinct but representative land-shells, some of which live in
crevices of stone; and although large quantities of stone are annually
transported from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species: nevertheless both islands
have been colonised by some European land-shells, which no doubt had some
advantage over the indigenous species. From these considerations I think
we need not greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as in
the several districts of the same continent, pre-occupation has probably
played an important part in checking the commingling of species under the
same conditions of life. Thus, the south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.

The principle which determines the general character of the fauna and flora
of oceanic islands, namely, that the inhabitants, when not identically the
same, yet are plainly related to the inhabitants of that region whence
colonists could most readily have been derived, -- the colonists having been
subsequently modified and better fitted to their new homes, -- is of the
widest application throughout nature. We see this on every mountain, in
every lake and marsh. For Alpine species, excepting in so far as the same
forms, chiefly of plants, have spread widely throughout the world during
the recent Glacial epoch, are related to those of the surrounding
lowlands; -- thus we have in South America, Alpine humming-birds, Alpine
rodents, Alpine plants, &c., all of strictly American forms, and it is
obvious that a mountain, as it became slowly upheaved, would naturally be
colonised from the surrounding lowlands. So it is with the inhabitants of
lakes and marshes, excepting in so far as great facility of transport has
given the same general forms to the whole world. We see this same
principle in the blind animals inhabiting the caves of America and of
Europe. Other analogous facts could be given. And it will, I believe, be
universally found to be true, that wherever in two regions, let them be
ever so distant, many closely allied or representative species occur, there
will likewise be found some identical species, showing, in accordance with
the foregoing view, that at some former period there has been
intercommunication or migration between the two regions. And wherever many
closely-allied species occur, there will be found many forms which some
naturalists rank as distinct species, and some as varieties; these doubtful
forms showing us the steps in the process of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way. Mr.
Gould remarked to me long ago, that in those genera of birds which range
over the world, many of the species have very wide ranges. I can hardly
doubt that this rule is generally true, though it would be difficult to
prove it. Amongst mammals, we see it strikingly displayed in Bats, and in
a lesser degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many
individual species have enormous ranges. It is not meant that in
world-ranging genera all the species have a wide range, or even that they
have on an average a wide range; but only that some of the species range
very widely; for the facility with which widely-ranging species vary and
give rise to new forms will largely determine their average range. For
instance, two varieties of the same species inhabit America and Europe, and
the species thus has an immense range; but, if the variation had been a
little greater, the two varieties would have been ranked as distinct
species, and the common range would have been greatly reduced. Still less
is it meant, that a species which apparently has the capacity of crossing
barriers and ranging widely, as in the case of certain powerfully-winged
birds, will necessarily range widely; for we should never forget that to
range widely implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the struggle for
life with foreign associates. But on the view of all the species of a
genus having descended from a single parent, though now distributed to the
most remote points of the world, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very
widely; for it is necessary that the unmodified parent should range widely,
undergoing modification during its diffusion, and should place itself under
diverse conditions favourable for the conversion of its offspring, firstly
into new varieties and ultimately into new species.

In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have been
ample time for great climatal and geographical changes and for accidents of
transport; and consequently for the migration of some of the species into
all quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to believe
from geological evidence that organisms low in the scale within each great
class, generally change at a slower rate than the higher forms; and
consequently the lower forms will have had a better chance of ranging
widely and of still retaining the same specific character. This fact,
together with the seeds and eggs of many low forms being very minute and
better fitted for distant transportation, probably accounts for a law which
has long been observed, and which has lately been admirably discussed by
Alph. de Candolle in regard to plants, namely, that the lower any group of
organisms is, the more widely it is apt to range.

The relations just discussed, -- namely, low and slowly-changing organisms
ranging more widely than the high, -- some of the species of widely-ranging
genera themselves ranging widely, -- such facts, as alpine, lacustrine, and
marsh productions being related (with the exceptions before specified) to
those on the surrounding low lands and dry lands, though these stations are
so different -- the very close relation of the distinct species which inhabit
the islets of the same archipelago, -- and especially the striking relation
of the inhabitants of each whole archipelago or island to those of the
nearest mainland, -- are, I think, utterly inexplicable on the ordinary view
of the independent creation of each species, but are explicable on the view
of colonisation from the nearest and readiest source, together with the
subsequent modification and better adaptation of the colonists to their new
homes.

Summary of last and present Chapters -- In these chapters I have
endeavoured to show, that if we make due allowance for our ignorance of the
full effects of all the changes of climate and of the level of the land,
which have certainly occurred within the recent period, and of other
similar changes which may have occurred within the same period; if we
remember how profoundly ignorant we are with respect to the many and
curious means of occasional transport, -- a subject which has hardly ever
been properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that all the
individuals of the same species, wherever located, have descended from the
same parents, are not insuperable. And we are led to this conclusion,
which has been arrived at by many naturalists under the designation of
single centres of creation, by some general considerations, more especially
from the importance of barriers and from the analogical distribution of
sub-genera, genera, and families.

With respect to the distinct species of the same genus, which on my theory
must have spread from one parent-source; if we make the same allowances as
before for our ignorance, and remember that some forms of life change most
slowly, enormous periods of time being thus granted for their migration, I
do not think that the difficulties are insuperable; though they often are
in this case, and in that of the individuals of the same species, extremely
grave.

As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long course
of time the individuals of the same species, and likewise of allied
species, have proceeded from some one source; then I think all the grand
leading facts of geographical distribution are explicable on the theory of
migration (generally of the more dominant forms of life), together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water, which
separate our several zoological and botanical provinces. We can thus
understand the localisation of sub-genera, genera, and families; and how it
is that under different latitudes, for instance in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and
deserts, are in so mysterious a manner linked together by affinity, and are
likewise linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relations of organism to
organism are of the highest importance, we can see why two areas having
nearly the same physical conditions should often be inhabited by very
different forms of life; for according to the length of time which has
elapsed since new inhabitants entered one region; according to the nature
of the communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each other
and with the aborigines; and according as the immigrants were capable of
varying more or less rapidly, there would ensue in different regions,
independently of their physical conditions, infinitely diversified
conditions of life, -- there would be an almost endless amount of organic
action and reaction, -- and we should find, as we do find, some groups of
beings greatly, and some only slightly modified, -- some developed in great
force, some existing in scanty numbers -- in the different great geographical
provinces of the world.

On these same principles, we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but of these a great
number should be endemic or peculiar; and why, in relation to the means of
migration, one group of beings, even within the same class, should have all
its species endemic, and another group should have all its species common
to other quarters of the world. We can see why whole groups of organisms,
as batrachians and terrestrial mammals, should be absent from oceanic
islands, whilst the most isolated islands possess their own peculiar
species of aerial mammals or bats. We can see why there should be some
relation between the presence of mammals, in a more or less modified
condition, and the depth of the sea between an island and the mainland. We
can clearly see why all the inhabitants of an archipelago, though
specifically distinct on the several islets, should be closely related to
each other, and likewise be related, but less closely, to those of the
nearest continent or other source whence immigrants were probably derived.
We can see why in two areas, however distant from each other, there should
be a correlation, in the presence of identical species, of varieties, of
doubtful species, and of distinct but representative species.

As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the exceptions to the rule are so few, that they
may fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period
under different conditions or by occasional means of transport, and by the
species having become extinct in the intermediate tracts. Both in time and
space, species and groups of species have their points of maximum
development. Groups of species, belonging either to a certain period of
time, or to a certain area, are often characterised by trifling characters
in common, as of sculpture or colour. In looking to the long succession of
ages, as in now looking to distant provinces throughout the world, we find
that some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of the
same order, differ greatly. In both time and space the lower members of
each class generally change less than the higher; but there are in both
cases marked exceptions to the rule. On my theory these several relations
throughout time and space are intelligible; for whether we look to the
forms of life which have changed during successive ages within the same
quarter of the world, or to those which have changed after having migrated
into distant quarters, in both cases the forms within each class have been
connected by the same bond of ordinary generation; and the more nearly any
two forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have been
the same, and modifications have been accumulated by the same power of
natural selection.